As a preparatory step in the manufacturing of fibro-cement articles, it is required to prepare a cement paste containing segments of reinforcing fiber uniformly distributed in the paste. The strength properties in a given reinforcing fiber have of course remarkable effect on the end properties of the structures made from the paste of cementing material containing the fiber incorporated therein. The degree of adherence between the paste of cementing material and the individual fibers has also extreme influence on the structure properties. The preservation of the strength properties of the fiber in the process of its incorporation, as well as the adherence degree attainable in the end structure largely depend on the process followed for incorporating the fibers of reinforcing material into the matrix of cementing material pastes.
A method for incorporating segments of fibrous material into a cement paste, in practice at the present, is a mixing process. In this process the components, cement, water, inert filler material and plastizicer agent are placed into a mixer. The suitable mixer may be a rotary drum interiorly fitted with blades arranged so as to agitate the plastic mass, or it may be a container equipped with a mechanical stirrer. An alternative process is pneumatically incorporating the reinforcing fibers into the previously mixed cement paste. In the latter case a paste jet under a substantial pressure, is combined with pneumatic jet containing the fibrous reinforcing material. The union of both of the pressure jets originates the mass comprised of the fibrous material and the cement paste, in a more or less homogeneous condition.
In either of the foregoing processes it is necessary to improve the fluidity of the cement paste by the use of a fluidifying agent. High degree of fluidity will enhance wetting of the fiber by the cement paste; as a result the attainable adherence under these conditions will be good. In order to achieve the desirable degree of fluidity, however, an amount in excess of fluidifying agent is frequently used. Since this excess amount of fluidifying agent adversely affects the fiber reinforcing properties, which tends to offset the enhanced effect on the adherence property obtained as a consequence of improving the initial wetting of the fiber, it is therefore necessary to remove the excess amount of said fluidifying agent from the final paste. This is a cumbersome removal operation which is carried out with substantial difficulty, and in addition, it is of effect on the cost of the process for incorporating reinforcing fibrous material into the cement paste.
In either of the aforecited prior art processes, the fibrous segments are subjected to impact or to an excess of friction, to a greater or lesser extent. If a fracture is initiated in the fiber or if fiber is actually broken, then as a result, the possibility of completely using the full fiber strength is decreased. The fracture frequently shortens the fiber length to such an extent where is not long enough so that, through the physical adherence between the cement matrix and the fiber, the fiber can not be stressed up to the limit of its stress capacity. This is so because the stress that can be developed in a fiber is directly proportional to the fiber length which is adhered to the stress-transmitting matrix. As a consequence, it is desirable to have a process for incorporating fibrous segments into the cement paste, wherein the original physical properties that the fiber had before its incorporation, be as far as possible preserved in the final paste.
Furthermore, the adherence that ultimately may be attained between the cementing material matrix and the fibrous material segments is affected by the degree of fiber opening. Cement-mass reinforcing fibers, either natural or man-made fibers, are often in multifilamentary strands. In the strand condition, a great number of filaments fail to be suitably wetted by the cement paste because they are so packed together that the paste cannot enter the filamentary strand. It is therefore important that the process for incorporating fibrous material into the cement paste brings about a suitable degree of opening of the fibrous strand or spreading out of the individual filaments, and this is to be done under conditions not originating fiber breakage. Further, normally the fibrous strands have been treated with binders or size. In this respect, the mixing process for incorporation of fibers into the cement paste, allows suitable fiber opening, unless the fibers used have strong binders. In any case, however, there is the danger of fiber breakage, even without opening. As far as the process using pneumatic means for incorporating fibrous material into cement paste is concerned, this process does not facilitate to a substantial extent the fiber strand opening; it only induces incipient opening of the fibers, if weak binders are used. This in addition accounts for the fact that, when this process for incorporating fibers is used, the adherence of fibers to the cementing matrix, is more or less poor.
Finally, an excessive amount of entrained air into the cement paste, impairs the adherence between fibers and matrix, and as a result the reinforcing properties are also adversely affected. In the process of incorporation of fiber by mixing operation, a great amount of air is left occluded into the cementing matrix since, in the process conditions, no particular means are provided permitting the air to migrate to the mass surface. For the air bubbles to reach the surface, they would have to travel substantial distances. In consequence a great number of air bubbles remain entrained between fibers and cement paste, which remarkably decreases the adherence between the fibers and the mass. With regard to the process using pneumatic means for incorporating segmented-fibrous material into the cement paste, part of the air used for forming both the paste jet and the fiber jet, is left entrained in the final mass. The result is an end mass having a great amount of air. This additionally accounts for the decreased adherence achieved by means of this process, and consequently, the use of the fiber is poor, and its reinforcing action is greatly diminished.